Occlusion pressures during the ventilatory response to hypoxemia in the newborn monkey.

End-expiratory airway occlusions were performed in eight unanesthetized premature newborn monkeys during acute hypoxemia to investigate mechanisms involved in the newborn's biphasic ventilatory response to hypoxia. Two-day-old monkeys demonstrated an immediate increase in minute ventilation (VI) and a decrease in PaCO2 followed within 5 min by a return of VI and PaCO2 to base-line levels. The decline in VI was associated with a decrease in tidal volume (VT) and inspiratory flow (VT/TI) and an increase in respiratory frequency. Occlusion pressures (PO.2) remained elevated throughout the hypoxic stimulus, and end-expiratory lung volume increased during the late response. "Effective" impedance (P0.1/V0.1, P0.2/V0.2, etc.) and "effective" elastance (Pmax/VT) were also elevated. At 21 days of age, the monkeys demonstrated a sustained ventilatory response as VI, VT, VT/TI, and P0.2 remained elevated throughout the period of hypoxemia. End-expiratory lung volume increases as on day 2, but effective impedance and effective elastance did not change. These data suggest that the biphasic response to hypoxia in the newborn may result from a change in respiratory timing and an alteration in respiratory mechanics and is not due to a decrease in central respiratory drive.